Adsorption systems, such as HVAC systems, liquid and gas purification, solvent and gasoline vapor recovery and deodorization, sorption cooling processes, certain bulk gas separations, etc., sometimes use adsorption media to remove gas phase impurities or more strongly adsorbed major components in a gas mixture. Adsorption processes and sorption cooling processes typically employ some adsorbent media disposed in a metal vessel, which may be self-supporting or contained on a metal screen or surface. The adsorbent is in contact with a fluid or gas stream containing an adsorbable component over the range of conditions necessary for adsorption.
Cyclic adsorption processes are frequently used to separate the components of a gas mixture. Typically, cyclic adsorption processes are conducted in one or more adsorbent vessels that are packed with a particulate adsorbent material that adsorbs at least one gaseous component of the gas mixture more strongly than it adsorbs at least one other component of the mixture. The adsorption process comprises repeatedly performing a series of steps, the specific steps of the sequence depending upon the particular cyclic adsorption process being carried out.
In any cyclic adsorption process, the adsorbent bed has a finite capacity to adsorb a given gaseous component and, therefore, the adsorbent requires periodic regeneration to restore its adsorption capacity. The procedure followed for regenerating the adsorbent varies according to the process. In VSA processes, the adsorbent is at least partially regenerated by creating vacuum in the adsorption vessel, thereby causing adsorbed component to be desorbed from the adsorbent, whereas in PSA processes, the adsorbent is regenerated at a lower pressure than the pressure used for the adsorption step. In both VSA and PSA processes, the adsorption step is carried out at a pressure higher than the desorption or regeneration pressure.
Some conventional adsorption media are comprised of a thin sheet or layer such as paper, metal foils, polymer films, etc., and an adsorbent material such as silica gel, activated alumina, activated carbon and molecular sieves such as zeolites. These adsorbent sheets or layers are relatively thin compared to conventional beads, extrudates, or granules. Because thinner media provides a shorter path length from the gas or liquid phase feed to the adsorption site, the mass transfer through these adsorbents is faster than in beads or granules. In addition, the macropore size distribution, particularly in wet laid adsorbent-containing paper, can be roughly an order of magnitude larger than in a typical adsorbent bead. This larger macropore size also increases the mass transfer of the media relative to beads or granules.
Rapid cycle processes require a gas permeable paper that will adsorb and desorb gaseous components in a rapid fashion. Although zeolite-containing papers are known to be used in such processes, such papers often lose their ability to adsorb and desorb the gases because the zeolite particles become detached from the paper, rendering the paper ineffective. This is due to the nature of the rapid cycle process whereby the direction of rapid (high velocity) gas flow is changed repeatedly.